<header>SMTP server response on unknown client reject</header>

<center><tt>unknown_client_reject_code</tt></center>
<hr>

This parameter specifies the SMTP server response code when a client without address
to name mapping violates the <tt>reject_unknown_clients</tt> restriction rule that is:<br>
<i>Access denied: Unknown SMTP client hostname (without DNS A or MX record)</i><br>
<br>
Note that the postfix default value used for <tt>unknown_client_reject_code</tt> is
code 450 also used for other errors. This code in 'standard RFC 822' means
'Requested mail action not taken: mailbox busy or access denied' and
ask the sender to try again later (hoping the DNS will answer correctly
later).<br>
<p>
Do not change this unless you have a complete understanding of RFC 822.
</p>

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